Do small magnitude, anthropogenic earthquakes induce pore pressure changes in the subsurface?

Abstract:

An increase in seismicity in south-central and central Kansas (KS), starting in 2013, has been linked to elevated pore fluid pressures in the subsurface resulting from high rate wastewater injections. Improving the understanding of how pore pressure is being transferred at local scales (~10 km) through the shallow basement (upper 10 km in the subsurface) can help mitigate the risk of induced seismicity. Previous research has found that fluid levels in wells are sensitive to large magnitude regional and teleseismic earthquakes. Here, we examine whether small local seismic events can induce detectable pore fluid pressure changes. We analyze pore pressure measurements made ~30 m above basement in a well at Wellington oil field in KS, USA. Our first recorded evidence of earthquake induced pore pressure changes comes from the magnitude 5.8 Pawnee, Oklahoma earthquake, 115 km from the monitoring well. In order to compare pore pressure measurements to earthquake signals, the pressure data is filtered to remove noise, oversampled to artificially increase the sampling rate from recorded 1 Hz data, and the derivative is taken to simulate an oscillatory waveform time series. The derivative of the pressure is analyzed with an STA/LTA trigger algorithm, analogous to earthquake detection methods. The triggers produced from this algorithm are compared to earthquake data collected at the field by a fifteen station monitoring network surrounding the pressure sensor borehole.

It is hypothesized that small magnitude local earthquakes will induce pore fluid pressure changes. Not all earthquakes will cause detectable pore pressure changes due to the geologic heterogeneity of the subsurface influencing pressure communication, as well as varying earthquake characteristics. Preliminary results, using the trigger algorithm, have identified numerous pressure events related to earthquakes in the month of November 2016. These detected pressure events correspond to earthquakes ranging in magnitude from 1 to 5 at distances of 3 to 150 km from the monitoring well. Ongoing analysis is conducted to determine the effect of the earthquake signals and their locations. By using downhole pressure and earthquake data from the field, it will be possible to better understand how small seismic events relate to changes in subsurface fluid pressure.